1. Field of the Invention
This invention relates to hot rolled steel sheets, cold rolled steel sheets and surface treated steel sheets having not only improved formability for press forming, deep drawing or the like but also improved fatigue resistance at a welded joint.
2. Related Art Statement
In general, the thin steel sheets are widely used for press forming, deep drawing and the like. However, it is required to have properties in accordance with use purposes in addition to the above formability. For example, the thin steel sheets are frequently subjected to a welding, particularly, spot welding irrespective of cold rolled sheets, hot rolled sheets and surface treated sheets.
Particularly, the thin steel sheet is used for automobiles. In this case, the spot number in the spot welding per one vehicle amounts to several thousand points and also stress concentration is apt to caused in the welded joint portion when a load is applied from exterior. That is, the fatigue breakage through the repetition of such a stress concentration during the running of the vehicle is caused in the welded joint portion, resulting in the occurrence of serious accidents. In the formable thin steel sheet, therefore, the fatigue resistance of the welded joint is a very important characteristic.
On the other hand, extreme-low carbon steels having a formability higher than that of the conventional low carbon steel are frequently used for the thin steel sheet. However, the fatigue strength of the extreme-low carbon steel may be lowered due to poor texture of heat-affected zone in the welded joint in accordance with the conditions.
Moreover, it is demanded to more improve the safety of machines and structures such as automobiles and the like as a worldwide theme, and consequently it becomes significant to enhance the fatigue strength of the welded joint as compared with the case of using the conventional steel sheets.
In this connection, there are proposed various steel sheets in Japanese Patent laid open No. 54-135616, No. 53-52222, No. 61-246344, No. 58-25436, No. 53-137021, No. 58-110659 and the like. However, all of these techniques disclose the mechanical properties of the cold rolled steel sheet but are silent in the fatigue strength of the welded joint.
Furthermore, Japanese Patent laid open No. 63-317625 discloses a method of controlling amounts of Ti, Nb and B to particular ranges for improving the fatigue resistance of the welded joint in the steel sheet. In this method, however, the tensile shear fatigue properties in the spot welded zone are considered, but there is no consideration on the cross tensile fatigue properties. Moreover, Japanese Patent laid open No. 225748 discloses cold rolled steel sheets having excellent fatigue properties, but in this case the fatigue properties of the sheet itself are merely improved.